A Pirani sensor uses a measuring element suspended in a tube which is connected to the system whose vacuum is to be measured. The measuring element is typically a heated metal wire (also called a filament). A filament suspended in a gas will lose heat to the gas as the gas's molecules collide with the wire and remove heat. If the gas pressure is reduced the number of molecules present will fall proportionately and the wire will lose heat more slowly. Measuring the heat loss is an indirect indication of pressure. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken.
Exemplary Pirani sensors are disclosed in German patent no. DE19903010 and in European patents No. EP0660096 and EP1409963. Further exemplary sensors and their operating modes are disclosed in U.S. Pat. Nos. 7,360,415; 7,497,118; 7,642,923 and 8,047,711 which are hereby incorporated herein by reference in their entireties.
While the heat loss from the filament into the gas is an indicator of the gas pressure, conventional Pirani sensors also experience conductive heat loss from the filament into the filament's suspension and radiation heat losses from the filament. During operation, the conductive heat loss into the suspension (Psuspension) and radiation heat loss (Pradiation) add up to a base power Pzero which is required to maintain the operating condition of the sensor. This base power may also be referred to as “zero pressure” p0, indicating the pressure that would lead to the same heat loss into the gas as the parasitic effects of conductive and radiation heat loss in a complete vacuum.
The base power Pzero of a Pirani sensor depends on the sensor's geometry, material properties, and environmental conditions in which the sensor operates, especially the ambient temperature. The material properties that affect base power include the emission coefficient of the measuring element (filament) surface and reflection properties of the surface into which power is transferred by radiation. While the influence of the geometry, for example the thickness of the heat dissipating suspension pins, and the influence of the material properties may be assumed to be design related constants, the influence of the ambient temperature is variable.
The greater the amount of parasitic heat losses and therefore the base power Pzero, the more difficult the detection of small changes in the thermal conductivity, heat capacity, pressure or flow of the measured fluid becomes.
Goal in the design of such sensors is therefore a minimization of base power Pzero. For reasons of mechanical stability, however, there is a tight limit to reducing the dimensions of the measuring element's suspensions. The suspension must be capable of carrying the measuring element, which may be a (metal) wire, measuring filaments, or a membrane that carries measuring elements.
Conventional approaches have attempted to compensate for changing ambient influences, such as ambient temperature, by additional measuring resistors in the electrical evaluation circuit to which the sensor is connected. Those approaches are, however, limited. Since the ambient influence on a Pirani sensor depends not only on the temperature but also on the pressure of the measured fluid, compensation by a temperature sensor is, strictly speaking, valid only for a single operating point.